With turbochargers required to have transient response, there is a demand for “an improvement in efficiency” for converting the exhaust energy into an increase in the suction pressure, and “an improvement in rotational acceleration” for reducing the “so-called turbo lag”, a delay in the power increase of an engine with a turbocharger.
Therefore, the efficiency of the compressor and the turbine has been improved, and the moment of inertia of the rotor has been reduced by reducing the size and weight of the turbine wheel, thereby improving the response of the turbo engine when accelerating.
Generally, in order to “improve the efficiency aerodynamically”, it is an effective approach, for example, to increase the number of blades to reduce the blade load, but it will increase the weight and increase the inertial mass, on the other hand, thereby resulting in a problem of a “decrease in the rotational acceleration”, and therefore there has been a demand for an approach capable of realizing both of these contradicting effects.
The present applicant has proposed a technique of a mixed flow turbine shown in Patent Document 1 as one that suppresses the turbine efficiency decrease, or one that suppresses the efficiency decrease in a mixed flow turbine in particular.
Referring to FIG. 17, a mixed flow turbine disclosed in Patent Document 1 will be described.
Provided is a mixed flow turbine 201 including: a hub 205 rotating about a central axis K; a plurality of rotor blades 207 provided standing on a hub outer circumferential surface 206 with its front edge 247 protruding toward the upstream side; a casing 213 having a shroud portion 227 covering a radial outer edge 225 of the rotor blade 207; and a scroll 223, which is a space formed on the upstream side of the rotor blade 207 for supplying a fluid toward the front edge 247 of the rotor blade 207, wherein the scroll 223 is divided by a scroll partition wall 229 into a shroud-side space 231 and a hub-side space 233.
Since a shroud-side partition wall surface 237 and a hub-side partition wall surface 235 on the rear edge side of the scroll partition wall 229 are provided with a shroud-side wall surface 243 and a hub-side wall surface 239 formed so as to oppose generally parallel thereto, respectively, there are formed, between respective wall surfaces, a shroud-side inflow passageway 245 where the fluid flows in a generally radial direction and a hub-side inflow passageway 241 where the fluid flows in a direction generally equal to the inclination direction on the hub side of the blade inlet.
Since the fluid supplied through this shroud-side inflow passageway 245 flows in a generally radial direction, the fluid flows in so as to be parallel to the shroud-side wall surface 243 and generally orthogonal to the inlet-side edge of the rotor blade. Therefore, at the shroud-side blade front edge of the mixed flow turbine rotor blade inlet, the flow can be guided into the rotor blade 207 at an appropriate flow angle.
Since the fluid supplied through the hub-side inflow passageway 241 is flowing in a direction generally equal to the inclination direction of the hub outer circumferential surface 206 of the mixed flow turbine rotor blade inlet, the fluid flows in so as to be parallel to the hub outer circumferential surface 206 and generally orthogonal to the blade front edge of the rotor blade. Therefore, at the hub-side blade front edge of the mixed flow turbine rotor blade inlet, the flow can be guided into the rotor blade 207 at an appropriate flow angle.
Since the flow coming from the hub-side inflow passageway 241 into the rotor blade 207 flows into the rotor blade 207 with an angle generally equal to the inclination of the hub outer circumferential surface 206, the flow through the shroud-side inflow passageway 245, which comes from the shroud-side inflow passageway 245 into the rotor blade 207 in a generally radial direction and is turned to the axial direction toward the rotor blade outlet, can be smoothly turned from the radial direction to the axial direction, thereby making it possible to prevent an increase in the wall surface boundary layer occurring in the shroud portion.
On the other hand, the fluid flows in a generally radial direction in the shroud-side inflow passageway 245, whereas the fluid flows in a direction generally equal to the hub-side inclination direction of the mixed flow turbine rotor blade inlet in the hub-side inflow passageway 241, and the fluids having passed through the inflow passageways flow into the inlet-side edge of the mixed flow turbine rotor blade while being orthogonal to each other.
Therefore, the fluids flowing in the shroud-side inflow passageway 245 and the hub-side inflow passageway 241 merge together at the rear edge of the scroll partition wall 229. Thus, it is possible to suppress the development of a wake occurring at the rear edge of the scroll partition wall 229.
Note that the mixed flow turbine having a turbine rotor blade with its front edge protruding toward the upstream side of Patent Document 1 is also disclosed in Patent Document 2.
Patent Document 1: Japanese Patent Application Laid-open No. 2009-281197
Patent Document 2: Japanese Patent No. 4288051